The communication system to be mounted on a car is conventionally adopted in which communication lines (hereinafter referred to as bus) connected to electronic control units (ECU: Electronic Control Unit) controlling the operation of appliances mounted on the car are connected with a relay connection unit such as a gateway to send and receive messages among the ECUs.
In recent years, with an increase in the function of the car, the number of the ECUs has increased and the number of messages to be sent and received among the ECUs has rapidly increased. Therefore to relay messages among the ECUs, the relay connection unit (so-called gateway) is interposed between buses connecting the ECUs to each other to efficiently send and receive the messages among the ECUs and decrease a communication load factor.
For example, as disclosed in Japanese Patent Application Laid-Open No. 2003-264571 (patent document 1), inside the ROM, there is provided the list routing table storing the correlation between the ID (identifier) of a message to be relayed to the relay connection unit and the bus through which the message is to be sent. With reference to the routing table, the message is relayed to the bus through which the message is to be sent, but is not relayed to other buses to decrease the communication load factor.    Patent document: Japanese Patent Application Laid-Open No. 2002-176430